Multiphase switching power supplies are well known and frequently used for high power applications. FIG. 1 illustrates a two-phase power supply. In such a multiphase power supply, a plurality of independently controlled switching power supplies are connected in parallel to drive a load, represented as a resistor RL, coupled to the regulated output voltage Vo. Typically, the controller 10 is implemented as an integrated circuit, and the inductors L1 and L2 and output capacitor Co are external. The switching transistors Q1-Q4 may be internal or external depending on the power requirements.
A clock sets each power supply during a different phase of a cycle by turning the top transistor Q1 or Q3 on at the beginning of its associated phase. In this way, the current conducted by each phase is only a fraction of the load current, and the output voltage ripple is reduced. This reduces the filtering requirements, reduces RMS power dissipation in the switches, reduces hot spots, enables more rapid response to load changes, and eases the requirements for traces on printed circuit boards and in integrated circuits. Ideally, the currents provided by the phases are the same under steady state conditions.
Current mode switching power supplies are commonly used in multiphase switching power supplies and require a very accurate current sensor to feed back the instantaneous inductor current to regulate the peak currents through the various inductors in the phases. Basically, when the ramping inductor current crosses a threshold voltage, the switching transistor is turned off for the remainder of the clock cycle. The current sensing should be identical for each phase to ensure the load current is balanced evenly across all the phases. FIG. 1 shows the current feedback signals Ifb1 and Ifb2 for each of the phases, and shows the output voltage feedback signal Vfb. The output voltage feedback signal Vfb may be a divided voltage.
One technique for detecting the inductor current in each phase is to insert a low value sense resistor (e.g., less than 0.1 ohm) in series with the inductor and measure the voltage drop across the resistor. The voltage drop includes a relatively large drop due to the DC component of the ramping inductor current and a much smaller drop due to the AC-ripple component of the ramping inductor current. Since the resistor in each phase has a very low value, there is a poor signal to noise ratio. The signal to noise problem is due to the relatively small ripple voltage (AC) drops across the sense resistor while the resistor is simultaneously conducting a high DC current and switching noise. Further, losses in the resistors waste power.
Instead of using a separate series resistor, current can be measured “losslessly” by sensing the voltage across the inductor (since the inductor has a DC winding resistance called DCR) or sensing the voltage across the synchronous rectifier switch (when it is turned on). This technique is considered lossless because it relies on resistive losses inherent in the converter topology.
Another way to effectively sense the current is to emulate the inductor current using a resistor-capacitor network across the inductor, where the time constant of the RC network is the same as the inductor-DCR time constant so that RC=L/DCR. Accordingly, the ramping voltage across the capacitor will track the ramping current through the inductor. However, if DCR is very low, there will be a switching noise problem and a signal to noise ratio problem. This will lead to pulse width modulation (PWM) phase jittering, current imbalances, and other issues.
U.S. Pat. No. 8,823,352 discloses various current sense techniques for a single-phase power supply, but does not address current sensing for a multiphase power supply. The '352 patent discloses a technique to separate out the AC and DC components of the inductor current, to effectively independently amplify the AC component, and then suitable amplify the DC component to have the proper proportion to the AC component. However, this technique has issues when applied to a multiphase power supply, since each phase would need a separate amplifier having exactly the same gain to similarly amplify its associated DC component, and it is difficult to form identical amplifiers for each of the phases. Providing the separate amplifiers also adds cost and size to the system.
What is needed is a multiphase switching power supply that uses current mode converter phases, where the current sensing for each phase can be made more accurate and identical for each phase.